"This is a last chance," the US president said in a statement on Friday. "In the absence of such an agreement, the United States will not again waive sanctions in order to stay in the Iran nuclear deal.

"And if at any time I judge that such an agreement is not within reach, I will withdraw from the deal immediately."

Also on Friday, the US imposed separate sanctions against 14 Iranian individuals and entities it accuses of rights abuses, censorship and support for weapons proliferators.

The US still maintains sanctions on Iran - independent of the nuclear deal - on matters such as terrorism, human rights and ballistic missile development.

What is the deal?

The landmark accord was signed between six global powers and Iran in 2015.

It saw Iran agree to reduce uranium enrichment activity drastically, dispose of its enriched uranium stocks and modify a heavy water facility so it could not produce material suitable for a nuclear bomb.

In return, decades of international and US nuclear-related sanctions were suspended, and the US president must sign a waiver suspending them every 120 days.

But Mr Trump has repeatedly criticised the deal - reached under his predecessor Barack Obama - as "the worst ever".

What does the US want?

Senior Trump administration officials briefed reporters on Friday to say the president will work with European partners to broker an agreement limiting Iran's ballistic missiles activities.

Mr Trump is prepared to back a modification to the existing deal if it was made permanent, one official said.

The president also wants the US Congress to amend a law on US participation in the nuclear deal, so that Washington could reimpose all sanctions if Iran breaches certain "trigger points".

This will involve negotiations between the US and its European allies rather than talks with Iran, the official said.

BBC state department correspondent Barbara Plett Usher is sceptical that any international agreement can be negotiated in 120 days.

She says Iran is not interested in brokering a new deal, so Mr Trump will have either to back down or walk away.

Déjà vu

Analysis by Jonathan Marcus, BBC diplomatic correspondent

Some four months ago when President Trump refused to re-certify the Iran deal he passed the baton to Congress while keeping the agreement in place.

This time he has put the ball in the Europeans' court expecting Britain, France and Germany to come up with some accord that will, as he sees it, fix the deal's shortcomings.

While they worry about Iran's missile programmes and its regional activities, the Europeans believe the nuclear accord itself is working well. Furthermore even if they could come up with a follow-on agreement in a ludicrously short space of time, there is simply no chance of Iran agreeing to it.

So in another four months we will be back exactly where we are now. Mr Trump will have the problematic task of either re-imposing sanctions - effectively killing the nuclear accord - or of having to extricate himself from a problem of his own making.

Is the current deal working?

In October, President Trump refused to certify that Iran was in compliance with the accord, accusing the country of "not living up to the spirit" of the pact.

However, UN inspectors have certified nine times that Iran has not breached the deal.

European Union foreign policy chief Federica Mogherini, who helped broker the agreement, said on Thursday that the accord is "making the world safer".

She said it was also "preventing a potential nuclear arms race in the region".

What does Iran say?

Iran said on Friday that Mr Trump was "maliciously violating" the deal.